From November 13, 1995 through December 31, 2011, 70 (23%) of all 301 patients in whom ADAMTS13 activity was measured had activity < 10% by either the FRETS or immunoblotting assay or both. The data describe the presenting clinical features of these 70 patients. Severe neurologic abnormalities were defined as transient focal abnormalities, seizures, stroke, or coma; minimal abnormalities included headache and minor mental status changes such as transient confusion. Acute renal failure was defined as an increased serum creatinine of ≥ 0.5 mg/dL/d for 2 consecutive days or a serum creatinine ≥ 4.0 mg/dL with dialysis. Renal insufficiency was defined as any serum creatinine value ≥ 1.5 mg/dL.²⁷